Acid History: As human activity continues to pump nearly 50-fold more CO ₂ into the atmosphere than any existing natural sources, the oceans absorb it. Over time, this vast quantity of excess oceanic CO ₂ is expected to decrease oceanic pH and have marked effects on calcifying marine species. Looking to the past for records of the consequences, other instances of ocean acidification in geologic history caused by large natural events, such as volcanism, may help predict the oceans' response to contemporary CO ₂ levels. Hönisch et al. (p. 1058 ) review the geological events that potentially altered oceanic pH, from the last deglaciation to the largest mass extinction in Earth's history. The current rate of anthropogenic CO ₂ input into the oceans is much faster than at any other instance in the past, but yet it is unclear whether or not future ocean pH will be significantly affected.

Paleoclimate data show that climate sensitivity is ~3°C for doubled CO₂, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6°C for doubled CO₂ for the range of climate states between glacial conditions and ice-free Antarctica. Decreasing CO was the main cause of a cooling trend that began 50 million years ago, the planet being nearly ice-free until CO fell to 450 ± 100 ppm; barring prompt policy changes, that critical level will be passed, in the opposite direction, within decades. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO₂ will need to be reduced from its current 385 ppm to at most 350 ppm, but likely less than that. The largest uncertainty in the target arises from possible changes of non-CO₂ forcings. An initial 350 ppm CO₂ target may be achievable by phasing out coal use except where CO₂ is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO₂ is not brief, there is a possibility of seeding irreversible catastrophic effects.

The evolution of Earth’s climate on geological timescales is largely driven by variations in the magnitude of total solar irradiance (TSI) and changes in the greenhouse gas content of the atmosphere. Here we show that the slow ∼50 Wm⁻²increase in TSI over the last ∼420 million years (an increase of ∼9 Wm⁻²of radiative forcing) was almost completely negated by a long-term decline in atmospheric CO₂. This was likely due to the silicate weathering-negative feedback and the expansion of land plants that together ensured Earth’s long-term habitability. Humanity’s fossil-fuel use, if unabated, risks taking us, by the middle of the twenty-first century, to values of CO₂not seen since the early Eocene (50 million years ago). If CO₂continues to rise further into the twenty-third century, then the associated large increase in radiative forcing, and how the Earth system would respond, would likely be without geological precedent in the last half a billion years.